"It was monstrous self deception," Mr O'Toole said to Tsarnaev as he sentenced him to death. "To accomplish it, you had to forget your own humanity."

A Boston jury last month said Tsarnaev should be put to death for his role in setting off bombs at the finish line of the Boston Marathon on April 15, 2013.

Tsarnaev had previously been found guilty on all 30 charges he faced, 17 of which could bring the death penalty. In the sentencing phase, the same jury had to decide whether to sentence Tsarnaev to death or to send him to prison for the rest of his life.

Three people were killed in the 2013 bombing and more than 260 were injured.

After the bombing, a subsequent manhunt shut down Boston and grabbed the nation's attention. Tsarnaev eventually was found bloody inside a boat in the backyard of a residential neighbourhood.